1. Field of the Invention
The present invention relates to a wavelength tracking apparatus and method in a wavelength-division multiplexed (WDM)-passive optical network (PON), and more particularly, to a reliable WDM-PON system by aligning wavelengths of an optical source in the central office, a pass band of multiplexer/demultiplexer in the central office, and an optical source in the subscriber terminal, with respect to a pass band of the multiplexer/demultiplexer in the remote node, which varies according to ambient temperature.
2. Description of the Related Art
A digital subscriber line (DSL) technique that uses a unshielded twisted pair (UTP) and a cable modem termination system (CMTS) technique that uses a hybrid fiber coaxial (HFC), which have been currently used, are not expected to guarantee a bandwidth and service quality enough to provide subscribers with a convergence service of voice, data, and broadcasting which will be widely popularized in a few years. To solve this problem, a great deal of research has been conducted all over the world to develop a fiber-to-the home (FTTH) technique that connects the subscriber's home to the network via an optical fiber.
In a wavelength-division multiplexed (WDM)-passive optical network (PON), since communications are established between a central office and each subscriber by using a wavelength allocated to the subscriber, it is possible to provide a variety of independent communication services to each subscriber while guaranteeing quality of service and security. Also, unlike time division multiplexing (TDM), the WDM-PON assigns each wavelength to an individual subscriber who may use an optical source with low output power and a receiver with a narrow bandwidth.
However, the WDM-PON employs optical sources corresponding to subscribers, each optical source having a unique wavelength, thus increasing installation costs, and is substantially difficult to be competitive in cost over the TDM based passive optical network accordingly. Thus, development of a low-cost optical source for the WDM-PON is critically important. Also, in terms of equipment management, preparing a stock of optical sources having different wavelengths for respective subscribers against mechanical and functional troubles may be too heavy a burden for a service provider. Therefore, it is very important to design a WDM-PON that can provide subscribers with the ONT (optical Network Terminal) of one kind with wavelength-independent optical source.
For reliable management of the WDM-PON, it is important to monitor wavelengths of optical sources against aging of the componets or temperature changes, and optical fiber cut, and to align wavelengths of the multiplexer/demultiplexer whose pass band change according to ambient temperature.
In particular, it is very important to align wavelengths of optical sources and the multiplexer/demultiplexer in the central office, and an optical source of a subscriber terminal (ONT) with respect to a pass band of the multiplexer/demultiplexer in the remote node (RN) whose pass bands vary on ambient temperature changes.
For easy repair and management of the WDM-PON, electric current is not supplied to a remote node. However, in this case, the temperature of the optical multiplexer/demultiplexer in the remote node may change from −40° C. to 80° C., and particularly, to a maximum of 120° C., according to ambient temperature.
Accordingly, misalignment of wavelengths of the WDM multiplexers/demultiplexers (WMD) of the central office (CO) and the remote node (RN), and wavelengths of the WDM multiplexer/demultiplexer (WMD) in the remote node and each of optical sources of ONTs, may cause not only optical loss in the optical channels but also performance degradation due to crosstalk occurring between optical channels.
To solve these problems, a wavelength tracking method has been introduced to equalize a wavelength of an optical source for downward transmission with a passband of WMD, which varies upon ambient temperature change.
Also, a method has been introduced to equalize a passband of WMD in the RN with that of WMD in the CO for a WDM-PON that uses a spectrum-sliced optical source. However, these methods do not disclose alignment of the wavelength of an optical source, a pass band of WMD in CO, a pass band of WMD in RN, and an optical source in ONT. These methods are not applicable to a WDM-PON that uses a general single-mode optical source.